The present invention generally relates to prism optics and an optical information processing system incorporating such prism optics. More particularly, the invention concerns composite prism optics, which are suited for changing the shape of a light beam having a two-dimensional distribution as well as an optical information processing apparatus in which the prism optics are used.
As the optical system for changing the cross-sectional shape of a light beam, there have hithertofore been known two types of optical systems, i.e. an optical system in which a pair of lenses (cylindrical lenses) are disposed at their a focal point, where magnification power of the optical system is adjusted in dependence on the ratio of the focal length of the lenses, and an optical system in which a triangle prism is used and a difference existing between the angle of incidence and the exit angle of the light beam due to refraction of the prism is utilized for changing the shape of the light beam. The present invention concerns an improvement of the last mentioned type of optical systems.
FIG. 1 of the accompanying drawings illustrates refraction of a light beam by a triangle prism 1; in which a light beam is incident on one face of a triangle prism along a direction oblique to the one face. Referring to the figure, when the incident angle (i.e. angle of incidence) and the exit angle at the boundary between air and prism media are represented by .theta..sub.1 and .theta..sub.2, respectively, the following relation is valid (in accordance with the Snell's law). EQU sin .theta..sub.1 =n.multidot.sin .theta..sub.2 ( 101)
where n represents the refractive index of the medium constituting the triangle prism.
Further, variations of the diameter of the light beam brought about by the refraction is given by EQU D.sub.2 /D.sub.1 =cos .theta..sub.2 /cos .theta..sub.1, (102)
where D.sub.1 and D.sub.2 represent the diameters of the incident beam and the exit beam, respectively. By making use of the relation mentioned above, change of the beam shape can be realized. On a prism face opposite to the oblique one, the light beam is incident or exits in a direction perpendicular to said other prism face, as will be seen in FIG. 1, so that the ratio of beam diameter generated at the oblique face undergoes no change at said other face. More in detail, refer e.g. to U.S. Pat. No. 4,333,173.
The relations mentioned above are invariable so far as the wavelength of the light beam remains constant. However, when the wavelength varies, the refractive index of the medium constituting the triangle prism undergoes variations, bringing about a corresponding change in the exit angle .theta..sub.2 of the light beam, which in turn exerts significant influences on instruments used in combination with the optical system, what is a great disadvantage. The variation in the wavelength of the light beam is ascribable to various causes such as fluctuations of the wavelength of the light emitted by a light source such as, for example, a semiconductor laser device, change in the wavelength of the emitted light in the course of time lapse, change of emission power, change in the ambient temperature and the like. There are two sorts of variations of the refractive index due to temperature variations, i.e. variations of the refractive index brought about by variations in the wavelength due to temperature variations of the semiconductor laser device (wavelength variations) and variations of the refractive index due to temperature variations of the triangle prism it-self.